1. Field of the Invention
The present invention relates to a rechargeable lithium battery having an improved anode with a coat comprising a film comprised of a specific metal oxide material having a specific standard electrode potential difference or electromotive force with respect to lithium and capable of intercalating or deintercalating lithium ions generated during battery reaction and which stably exhibits an excellent battery performance while depositing lithium such that granules of lithium are uniformly and substantially two-dimensionally grown in said coat or at interfaces of said coat without the generation of a dendrite (or a branched tree-like protrusion) of lithium upon operating charging and has a prolonged charging and discharging cycle life. The present invention also relates to a process for producing said anode and said rechargeable lithium battery.
2. Related Background Art
In recent years, heating of the earth because of the so-called greenhouse effect due to an increase of atmospheric CO.sub.2 has been predicted.
In the case of the steam-power generation, the amount of fossil fuel, represented by coal or petroleum, consumed for power generation in order to comply with a societal demand for increased power supply has been continuously increasing. Along with this, the amount of exhaust fumes from the steam-power generation plants has been continuously increasing, and accordingly the content of gases, such as carbon dioxide, has risen to cause a greenhouse effect. This results in providing an earth-warming phenomenon. In order to prevent said earth-warming phenomenon from further developing, there is a tendency of prohibiting the establishment of new steam-power generation plants in some countries.
Under these circumstance, there has been made proposed the conducting of so-called load leveling in order to effectively utilize the power generator, wherein rechargeable batteries are installed at general houses and surplus power unused in the night, that is, a so-called dump power, is stored in said rechargeable batteries. The power thus stored is supplied in the daytime when the power demand is increased, whereby the power generator is leveled in terms of the load therefor.
In recent years, there has been the development of electric vehicles which do not exhaust any air polluting substances are of low impact to the environment, and which are expected to replace gasoline-fueled vehicles and diesel-fueled vehicles. For such an electric vehicle, there is an increased demand for developing a high performance rechargeable battery with a high energy density which can be effectively used therein.
Other than this, there is an increased demand for developing a miniature, lightweight, high performance rechargeable battery usable as a power source for portable instruments such as small personal computers, word processors, video cameras, and pocket telephones.
Further, there is an increased demand for realizing a high performance rechargeable battery capable of attaining load leveling not only for power generated by solar cells but also for power generated by an aerogenerator or a wave activated power generator. The dump power can be stored in the rechargeable battery and the power thus stored can be supplied upon demand.
As such a rechargeable battery, there has been proposed various rocking chair type lithium ion batteries. In these batteries, a carbon material, such as graphite which is capable of intercalating lithium ions at an intercalation of its six-membered network plane provided by carbon atoms in the battery reaction upon charging is used as an anode material and a lithium intercalation compound capable of deintercalating said lithium ions from the intercalation in the battery reaction upon charging is used as a cathode material. Some of these batteries have been used practically. However, in any of these lithium ion batteries, the theoretical amount of lithium which can be intercalated by the anodes is only an amount of 1/6 per carbon atom. If it is desired to increase the amount of lithium intercalated by the anode above the theoretical amount, unavoidable problems will occur because lithium is often deposited in a dendritic state (that is, in the form of a dendrite) on the anode during the charging operation. Formation of lithium dendrites will cause internal-shorts between the anode and the cathode upon repeating the charging and discharging cycle, and a battery possessing a sufficient charging and discharging cycle life cannot be obtained.
Therefore, based on the constitution of the foregoing lithium ion battery, a desirable rechargeable battery having an electric capacity and energy density similar to those in a primary battery, in which a lithium metal is used as the anode cannot be realized.
There has been proposed a rechargeable lithium battery in which metallic lithium is used as the anode. However, such a rechargeable lithium battery has not yet been realized as practically usable because its charging and discharging cycle life is extremely short. A main reason for this short cycle life has been generally attributable to reactions of metallic lithium with impurities such as water or an organic solvent contained in an electrolyte solution, form an insulating film. The formation of the insulating film promoted the formation of a dendrite by metallic lithium during the charging operation, resulting in internal-shorts between the anode and cathode upon repeating the charging and discharging cycle. As a result, the charging and discharging cycle life of the rechargeable lithium battery is dramatically shortened.
Now, as above described, once the lithium dendrite is formed, the dendrite is liable to gradually grow upon the charging operation, resulting in causing internal-shorts between the anode and the cathode. When the anode is internally shorted with the cathode, the energy possessed by the battery is rapidly consumed at the internally shorted portion which causes additional problems e.g., the battery is heated or the solvent of the electrolyte is decomposed by virtue of heat to generate gas, resulting in raising the inner pressure of the battery. These problems result in damaging the rechargeable battery or/and shortening the lifetime of the battery.
There has been proposed a manner of using a lithium alloy such as lithium-aluminum alloy as the anode for a rechargeable lithium battery in order to suppress the reactivity of the lithium with water or an organic solvent contained in the electrolyte solution so that lithium dendrite formation is suppressed. However, this manner is not practical because the lithium alloy is difficult to shape into a spiral form and therefore, it is difficult to produce a spiral-wound cylindrical rechargeable battery; a desirable charging and discharging cycle life for a rechargeable battery cannot be obtained; and for a rechargeable battery that is obtained, a desirable energy density similar to that of a primary battery in which a metallic lithium is used as the anode cannot be realized.
By the way, Japanese Unexamined Patent Publication No. 13264/1988 (hereinafter referred to as document 1), No. 47381/1993 (hereinafter referred to as document 2) or No. 190171/1993 (hereinafter referred to as document 3) discloses a non-aqueous series rechargeable battery in which the anode is composed of a lithium alloy. Document 3 discloses a non-aqueous series battery directed to improving the cycle life and also in the cycle characteristics after battery storage. In this battery, the anode is composed of a material comprising an aluminum-manganese combined with a metal which is more electrochemically noble than aluminum such as vanadium, chromium, or titanium, and lithium as the anode active material. The active site of the alloy with lithium is increased to prevent localization of the reaction.
Further, Japanese Unexamined Patent Publication No. 114057/1988 (hereinafter referred to as document 4) discloses a non-aqueous series rechargeable battery aiming at an improvement in the charging and discharging characteristics. The anode of this battery is composed of a basic constituent comprising a sintered body of a mixture composed of fibrous aluminum and fibrous metal incapable of being alloyed with lithium and a negative material comprising a lithium-aluminum alloy.
In addition, Japanese Unexamined Patent Publication No. 234585/1993 (hereinafter referred to as document 5) discloses a non-aqueous series rechargeable battery directed to minimizing the generation of a dendrite so that the charging efficiency is improved and the battery cycle life is prolonged. In this battery, the anode is composed of lithium metal, having powdery metal (which hardly forms an intermetallic compound with said lithium metal) uniformly deposited on the surface thereof.
However, any of the rechargeable batteries disclosed in the above documents 1 to 5 is still problematic because as the charging and discharging are alternately repeated over a long period of time, the anode is repeatedly expanded and shrunk and become damaged by loss of the constituents or by formation of a crack. The generation or growth of a dendrite cannot be sufficiently prevented and the rechargeable battery eventually shows poor current collecting performance.
Other than the above-mentioned documents, Journal of Applied Electrochemistry, 22, 620-627 (1992) (hereinafter referred to as document 6) discloses a rechargeable lithium battery in which the anode is constituted by an aluminum foil having a surface applied with etching treatment. However, the rechargeable lithium battery disclosed in the document 6 is also problematic. When the charging and discharging cycle is repeated for as many times as practically conducted for the ordinary rechargeable battery, problems are liable to occur, such that when the charging and discharging are alternately repeated, the aluminum foil is repeatedly expanded and shrunk and becomes damaged by a crack, resulting in causing a reduction in the current collecting performance. The growth of a dendrite is also liable to occur.
Hence, any of the rechargeable batteries disclosed in the documents 1 to 6 is still accompanied by some problems that remain to be solved.
Accordingly, there is an increased demand for the provision of an improved, highly reliable rechargeable lithium battery which is high in energy density (or charge energy density) and long in charging and discharging cycle life.
Aiming at eliminating the problems found in the conventional rechargeable lithium batteries, three of the four present inventors of the present invention together with another person have proposed a rechargeable lithium battery having an anode coated by an inorganic oxide film capable of allowing lithium ions generated during a battery reaction to pass through, in which the generation or growth of a dendrite of lithium is prevented, and this rechargeable lithium battery has an improved charging and discharging cycle life (see, Japanese Unexamined Patent Publication No. 283157/1994).